1. Field of the Invention
The present invention relates generally to optical devices (elements) and, more specifically, to the design of devices based on low-dimensional semiconductor structures.
2. Description of the Prior Art
Typically, quantum wells, quantum wires, and quantum dots (confined semiconductor structures) have abrupt interfaces that confine the electron and holes within. This abrupt confinement contributes to non-radiative Auger processes that severely limit the quantum efficiency of quantum well based devices (e.g., semiconductor lasers, light emitting diodes, semiconductor optical amplifiers, biological markers, etc.).
In the early sixties, the one-dimensional carrier confinement achieved in semiconductor quantum wells and superlattices brought about a revolution in solid-state device technology. To fulfill the requirements of miniaturization, low power consumption, and fast operational speed, further efforts of carrier confinement in two and in three dimensions were realized with the advent of quantum wires and quantum dots. However, the application of nanostructures to real world devices has been strongly curtailed by the enhancement of dissipative Auger processes which undergird all aspects of carrier relaxation and recombination. In particular, Auger processes have been attributed to the decrease of the photoluminescence quantum efficiency in light emitting diodes, an increase in the stimulated emission threshold in lasers, and the photoluminescence degradation and photoluminescence blinking in nanocrystal (NC) quantum dots. These detrimental effects were initially explained through other mechanisms since bulk wide-gap semiconductors have negligible Auger rates due to a temperature threshold proportional to their energy gap. Eventually, it was realized that the temperature threshold was not present because, unlike the requirement in the bulk case, Auger recombination in confined structures does not require a carrier with kinetic energy comparable to the energy gap. This explained why Auger processes are very efficient in confined structures, even those fabricated from wide-gap semiconductors.
Most impressively, Auger processes are visually manifest in the random intermittency observed in studies of the photoluminescence (PL) intensity emitted from a single NC. Even under constant illumination, all colloidal nanocrystals grown today exhibit this emission intermittency which has consequently been dubbed “photoluminescence blinking.” First observed about twelve years ago, the intermittency of the photoluminescence intensity came as a complete surprise in a study of a single CdSe under steady-state excitation conditions. Since then, many others have observed this effect at various temperatures in many other types of nanocrystals and nanowires. Today, the consensus is that the blinking occurs because, when illuminated, NCs can be charged (or ionized), and subsequently neutralized. Optical excitation of a neutral NC excites an electron-hole pair, which then recombines giving rise to the PL. However, if the NC is charged, the extra carrier triggers a process known as non-radiative Auger recombination during which the exciton energy is acquired by the extra charging electron or hole (see FIG. 1). Because the rate of Auger recombination is orders of magnitude faster than the rate of radiative recombination, photoluminescence is completely suppressed, or “quenched,” in charged NCs.